WR MORRIS HOT WIRE AMMETER, 1930's

Hot Wire Ammeters work the expansion and contraction of a piece of wire when heated by an electric current. Although not very accurate and prone to ageing effects of the wire, they were an inexpensive way of indicating currents of over 100 Milliamps.
Smaller current versions would have made the wire too fragile.

Your comments:

• Although 'not very accurate' one advantage of hot-wire ammeters was their ability to measure RMS of non DC currents to similar accuracy to that of DC.
  .......... Dave Kitchen, West Sussex, 6th of January 2022
  
  
• Hot wire ammeters have the advantage that they not only show DC currents, but they show true RMS for AC currents without modification. Subject to design, this could include RF current.
  .......... Simon Attwood, Aldershot, UK, 7th of November 2010
  
  

A0722